DFT study of single water molecule adsorption on the (100) and (101) surfaces of KH2PO4

Potassium dihydrogen phosphate (KH2PO4; KDP) is a widely used non-linear optical material. A critical issue associated with KDP crystals is that the water molecules in the environment inevitably adhere to the surface and affect the surface electronic and physical properties. In order to understand how water interacts with the surface, the adsorption of a single water molecule on the external surfaces of a KDP crystal, including (100) and (101) surfaces, was theoretically investigated based on density functional theory (DFT) method. The most favorable adsorption sites on both surfaces were considered based on the adsorption energy. The electron density difference and partial density of states were calculated to analyze the bond formation during the adsorption process. It was found that the water molecule can be adsorbed in two different ways-forming two hydrogen bonds with the O and H atoms on the surface or forming a hydrogen bond and an O-K chemical bond with the O and K atoms on the surface. We also found that the surface atoms and the pulling effect between two high potential states on the surface play a dominating role in the adsorption process.